The present invention relates to a clear holographic visor which serves to constitute a helmetmounted head-up navigational collimator and process for producing same. By means of this collimator, an aircraft pilot can view an image which is reflected at infinity and superimposed on his vision of the surrounding landscape. Depending on requirements, this light image can consist of navigation data, a sighting reticle or a synthetic image produced by a generator or an associated sensor such as an infrared sensor, for example.
As a general rule, the light image to be collimated is first formed on the screen of a cathode-ray tube or of a liquid crystal matrix, then directed to a first conventional optical system or a so-called optical relay which converts it to an intermediate image. The intermediate image was collimated by means of a partially reflecting optical system which can be composed of a flat plate and a spherical plate utilized on the axis. The two plates are treated in order to produce partial reflection of the rays received from the screen while partially transmitting those which originate from the landscape. This results in loss of information on both channels, namely on the imager channel and on the external landscape viewing channel. Moreover, the treatments employed are such that parasitic images such as, for example, reflections from the rear face of the flat plate, still remain and are liable to hinder the observer. A further drawback is that these collimators have a limited optical field and are too cumbersome to be mounted on a helmet.
In accordance with techniques which offer higher performances, an off-axis arrangement is adopted and the plates are subjected to holographic treatments for obtaining collimation with enhanced photometric efficiency by diffraction as well as improved transmission of the landscape channel. Solutions of this type are described in the article by J. R. Banbury published in the April 1983 issue of "Displays" and entitled "Wide field-of-view head-up displays". Particularly worthy of note in this article is the solution illustrated in FIG. 8C on page 95. This figure shows a head-up structure equipped with a reversing mirror and an optical relay system, the dimensions of which can be made compatible with the equipment by virtue of the fact that the observer's eye is placed at a sufficient distance from the combining and collimating assembly constituted by the flat plate and the spherical plate. In a system of this type, the observation pupil is accordingly placed in proximity to the center of the spherical mirror. The optical relay system serves to form the intermediate image as well as to correct aberrations of the holographic plates. By reason of the exit position of the pupil (at the center of the spherical mirror), this system cannot be perfectly optimized for anti-reflection treatments on the flat plate, with the result that it produces substantial parasitic images. Moreover, the use of an optical relay system is always penalized by the weight and bulk of the visor.
The object of the invention is to overcome these disadvantages by constructing a clear holographic visor of the type aforesaid having a wide field without any optical relay system, which can be mounted on a helmet, and which offers a very good photometric balance with a minimized proportion of parasitic images. Its limited overall size also makes it easier to mount on a helmet or within a cockpit.